Throughout the levels you come across some chests with a ? above them. Use the A to open them, and you will usually be given a random power-up. One of those power-ups is “Hobbe Disguise”. Once you get this, your character will change costume, and disguise himself as a hobbe. Now, just try to find an enemy that classifies as a hobbe, and kill them to unlock the achievement. Don’t worry if you missed out the first few times, you will get many chances throughout the game.
I personally got this in Gravestone, although it is available in many levels.
It also appears often during the Gravestone boss battle (choose the right-hand path).
Note: It is possible to get this power-up in sections with no hobbes.